Trigonometry: A Clever Study Guide is a book by James Tanton

James Tanton (a very interesting mathematician interested in HS student understanding):

Twitter: https://twitter.com/jamestanton

MAA: http://www.maa.org/math-competitions/teachers/curriculum-inspirations

YouTube: example (tetrahedron problem) ๐

James Tanton (PhD. Mathematics, Princeton University, 1994) is a research mathematician deeply **interested in bridging the gap between the mathematics experienced by school students and the creative mathematics practiced and explored by mathematicians**. He worked as a college professor for a decade, as a high-school teacher for a decade, and is now the Mathematician in Residence at the Mathematical Association of America in Washington D.C.

He often uses problems taken from American Mathematics Competitions (“AMC“) (wikipedia), which are very high quality problems.